In May of 1960, the largest earthquake ever recorded struck the coast of Chile with a force felt around the globe. The magnitude 9.5 temblor flattened villages, unleashed landslides, triggered a volcanic eruption, and launched tsunamis that ravaged shorelines in Hawaii, Japan, the Philippines, New Zealand, and California. Geologists could spend their entire careers reconstructing the seismic dynamics of the event. But in the past few years, they've become more interested in the calm before the terrestrial storm.
Fifteen to 20 minutes prior to the upheaval, a deep slip along an offshore fault occurred that didn't cause any shaking at all on the surface. Such movements, which last from minutes to years, are called slow, or silent, earthquakes, and only the most sensitive instruments can detect them. With technical advances such as GPS, slow earthquakes are now being detected at other infamous faults. They're providing clues to the makeup and behavior of deep-earth terrain that have long puzzled geologists. And they also hold out tantalizing prospects for forecasting earthquakes.
